When a large amount of data is outputted from a printer provided in an information processor, sheets of continuous-form paper are usually used, because there is little possibility of the occurrence of a paper jam or a failure in feeding sheets of paper. On the other hand, sheets of cut-form paper are used in usual office work. Various documents, the amount of data of which is small, are output on the sheets of cut-form paper. When a sheet of continuous-form paper is to be changed in the printing process, the sheet of paper is reversely fed. In this case, it is necessary to prevent the sheet of paper from slackening.
There is provided a paper feed mechanism of a printer in which a drive roller is mounted on a printer body, and an idle roller is mounted on a paper guide detachably attached to the printer body, and the idle roller comes into pressure contact with the drive roller so as to feed sheets of paper. Since the sheet of continuous-form paper is fed by a tractor, that is, by the action of pins, it is sufficient to give a weak force to the idle roller so as to pull the sheets of paper. When the pushing force is strong, sprocket holes are damaged and the tractor is disengaged from holes. When thick sheets of cut-form paper such as postcards are used, it is necessary to increase a pushing force of the idle roller so as to prevent the occurrence of a slippage. Concerning the pushing force of the idle roller, sheets of continuous-form paper and cut-form paper are incompatible with each other. Therefore, it is necessary to change over the pushing force according to the type of sheets of paper.
When a line feed operation is conducted in a conventional printer, in order to ensure a sheet passage for feeding sheets of paper stably, an amount of the gap formed between a printing head and a platen is increased to a predetermined value. However, when sheets of continuous-form paper are processed, on which perforations are formed in the boundary between pages, the perforations protrude from a surface of the sheet of paper. In this case, the printing head is caught by the perforations in the process of line feed, and the accuracy of line feed is deteriorated.
When sheets of cut-form paper are fed, sheets of paper stacked on the paper feed tray are sent out one by one. As a means for sending out sheets of paper one by one, there is provided a paper feed roller which comes into contact with a surface of the stacked sheet of paper and rotates to send them one by one. This type paper feed roller is commonly used. In a business machine, sheets of paper of various widths are used. Accordingly, in order to feed sheets of paper of various widths by one paper feed unit, a plurality of relatively short roller pieces are mounted on one roller shaft in accordance with the widths of the sheets of paper.
In the case of a printer, an OCR (optical character reader) or an image scanner in which the printing position and reading position of information must be accurately set, and all sheets of paper of different widths are guided by a reference side guide arranged at a stationary position on the paper feed tray. In this case, the positions of the roller pieces mounted on the roller shaft are determined in accordance with the widths of the sheets of paper while the reference side guide is used as a reference. Accordingly, intervals of the roller pieces mounted on the roller shaft are not equal. In the paper feed unit arranged as described above, when sheets of paper of the maximum width are fed, all roller pieces come into contact with the sheet surface. When sheets of paper of small width are fed, only the roller pieces within the width come into contact with the sheet surface. In order to feed sheets of paper by the above paper feed unit without causing a skew feed, it is necessary for the plurality of roller pieces to come into contact with the sheet surface with the same pressure.
The sheets of recording paper stacked on a hopper of the printer are fed one by one as follows. The uppermost sheet of recording paper in the stack is fed by the paper feed roller provided at an upper portion of the hopper. In order to prevent the second and later sheets of paper from advancing together with the first sheet of paper at this time, a separating pad is usually arranged opposite to the paper feed roller. When the second sheet of paper advances between the paper feed roller and the separating pad together with the first sheet of paper, the second sheet of paper is blocked by friction force due to the separating pad. In this way, the uppermost first sheet of paper is separated from the second sheet of paper and successively conveyed by the paper feed roller. Then the first sheet of paper is received from the paper feed roller by the conveyance roller, and a printing operation is conducted on the first sheet of paper by the printing section. After the completion of the printing operation, when a printing command is given to the second sheet of paper, the paper feed roller is rotated again, and paper feed operation is conducted in the same manner as that of the first sheet of paper. This operation is repeated and printing is conducted on a predetermined number of sheets of paper.
However, from a time when the first sheet of paper is sent to the printing section by the paper feed roller to a time when the second sheet of paper starts to be fed by the paper feed roller, a leading end of the next sheet of paper is interposed between the paper feed roller and the separating pad. Accordingly, when printing operation is continuously conducted on the next sheet of paper, no problems may be encountered. However, when the sheets of paper are replaced with sheets of paper of different size after the completion of a printing job, the hopper must be removed from the printer. In this case, it is difficult to pick up the sheet of paper, the leading end of which is interposed between the paper feed roller and the separating pad.
In general, when sheets of paper stacked on a tray are separated from each other and automatically fed one by one, a phenomenon of double feed tends to occur in the case where thin sheets of paper are fed, and a failure in feeding tends to occur in the case where thick sheets of paper are fed. When copy sheets composed of a plurality of layers are fed, layers tend to be separated from each other by an increased paper feeding force. Therefore, in order to prevent the occurrence of double feed and separation between layers, the conventional automatic paper feed unit adopts the following means. A paper feed force to be used as a reference force is set at a low value. When thick sheets of paper are fed, it is impossible to feed the sheets by the force of low intensity. In this case, the intensity of the paper feed force is increased.
However, in the conventional structure, the pushing force of the paper feed roller given onto sheets of paper is changed by a plurality of steps: Therefore, a plurality of springs are required for adjusting the pushing force of the paper feed roller, and it is necessary to set the spring force for each spring. When the paper feed resistance is high, the paper feed roller is raised due to the high paper feed resistance. Therefore, it is necessary to provide a space in which the paper feed roller can to be raised upward. When sheets of paper of high paper feed resistance are fed, the following operation is required. After the paper feed roller has been stopped, the paper tray is further raised, and then the paper feed roller is started again to feed the sheets of paper. Therefore, the throughput of sheets of paper is lowered.
In the case of an impact printer having a noise source, noise is mainly caused by an impact head for printing in the printing operation. This noise caused by the impact head leaks outside the printer from the sheet entrance. Therefore, an operator and others feel uncomfortable. In order to solve the above problems, it is necessary to reduce a leakage of noise from the printer as much as possible. In order to attain the above object, the following countermeasures are taken. The sheet feed entrance is made to be small; sound absorbing material is arranged around the sheet entrance; and the impact head is covered with sound absorbing material.
However, problems may be encountered by the above counter measures. When the sheet feed entrance is made to be small and sound absorbing material is arranged close to the sheet feed entrance, the sheet passage becomes narrow, so that the occurrence of a sheet jam tends to occur. In order to solve the above problems, the following countermeasures are taken. There is provided a sound insulating cover attached to the sheet feed entrance, and this cover is made to be opened and closed. Therefore, when sheets of paper pass through the sheet feed entrance, a clearance of the passage is reduced so as to prevent noise from leaking outside. In the case of a sheet jam, the sound insulating cover is opened for jam clearance. However, the above countermeasure is disadvantageous in that the structure is complicated, and a highly sophisticated operation is required.